Apparent size and judgment-order effect: A magnitude estimation study of the Delboeuf illusion

Apparent sizes of circles in the Delboeuf figure were measured by magnitude estimation without modulus. The purpose was to investigate the Delboeuf illusion and the effect of judgment order on perceived size. Contrary to the conventional Delboeuf illusion, overestimation effects appeared for outer circles at small diameter differences. The illusory magnitude depended on the diameter ratio and diameter difference of the constituent circles. The maximum magnitude of the illusion did not always appear at the diameter ratio, 0.67 (as found before by the method of limits). The circle judged first had an assimilative effect on the apparent size of the subsequently judged circle. The time-order dependency of assimilation was more pronounced for the inner than for the outer circle. The judgment-order effect may be accounted for by the attention mechanism.